Magneto resistance (MR) elements configured to exert magneto resistance effects are widely utilized as sensors for detecting a magnetic field because the electric resistance value of the MR elements vary in response to the external magnetic field. An example of such a sensor is a magnetic sensor including a bridge circuit with MR elements. Such magnetic sensors are disclosed in JP3017061B based on priority from U.S. Pat. No. 5,561,368A and JP2006-29792A.
FIG. 1 is a schematic diagram showing the configuration of the magnetic sensor described in JP3017061B. Magnetic sensor 100 includes first MR element 101, second MR element 102, third MR element 103, and fourth MR element 104.
First MR element 101 and second MR element 102 are connected together by an electrical conductor. Second MR element 102 and third MR element 103 are connected together by an electrical conductor. Third MR element 103 and fourth MR element 104 are connected together by an electrical conductor. Fourth MR element 104 and first MR element 101 are connected together by an electrical conductor. MR elements 101, 102, 103, and 104 form a bridge circuit.
Input leads 107 are connected to a point between second MR element 102 and third MR element 103 and to a point between fourth MR element 104 and first MR element 101. A voltage is applied between two input leads 107.
Output lead 108 is connected to a point between first MR element 101 and second MR element 102 and to a point between third MR element 103 and fourth MR element 104. The potential difference between two output leads 108 corresponds to an output from magnetic sensor 100.
Each of MR elements 101, 102, 103, and 104 includes a stack with ferromagnetic layer (free layer) 114 in which the magnetization direction varies in response to an external magnetic field, a magnetization fixed layer (pinned layer) 112 in which the magnetization direction is fixed with respect to the external magnetic field, a nonmagnetic intermediate layer (spacer layer) 113 sandwiched between magnetization fixed layer 112 and a ferromagnetic layer 114, and an antiferromagnetic layer (pinning layer) 111 configured to fix the magnetization direction in magnetization fixed layer 112. The stack is called a spin valve film. The magnetization in ferromagnetic layer 114 rotates freely in the film plane of the stack in response to the external magnetic field.
Magnetization direction 115 in the magnetization fixed layer in the first MR element is the same as magnetization direction 117 in the magnetization fixed layer in the third MR element. Furthermore, each of magnetization directions 115 and 117 in the magnetization fixed layers in the first and third MR elements is opposite to each of magnetization directions 116 and 118 in the magnetization fixed layers in the second and fourth MR elements.
In general, the electric resistance of the MR element varies depending on the angle between the magnetization direction in the magnetization fixed layer and the magnetization direction in the ferromagnetic layer. Since the magnetization direction in the ferromagnetic layer varies in response to the external magnetic field, the electric resistance of the MR element varies in response to the external magnetic field.
Thus, the potential difference (output value) between output terminal leads 108 in the bridge circuit varies depending on the direction of the external magnetic field. In the magnetic sensor, an output waveform corresponding to the rotation angle of the external magnetic field is generally shaped like a cosine or sine wave. The rotation angle of the external magnetic field as used herein refers to the angle (direction) of the rotation in the film plane of the stack of the MR element.
Since the output waveform from the magnetic sensor is estimated to be shaped like a cosine or sine wave, the direction of the external magnetic field can be calculated from the output value from the magnetic sensor.
To improve the accuracy of the magnetic sensor shown in FIG. 1, MR elements with a high magneto resistance ratio (MR ratio) are preferably utilized. An example of an MR element with a high MR ratio is a tunnel magneto-resistance element (TMR element) that utilizes a tunnel magneto-resistance effect. In the TMR element, the nonmagnetic intermediate layer includes an insulating layer. In the TMR element, a current is allowed to flow in a direction orthogonal to the film plane of the element. Thus, a tunnel current flows through the insulating layer.
However, the present inventors have clarified that the use of the TMR element may result in the following problems.
FIGS. 2A and 2B are graphs prepared by the present inventors. FIG. 2A shows the voltage dependence of the maximum and minimum values of the resistance of a TMR element. FIG. 2B shows the voltage dependence of the MR ratio of the TMR element. The resistance value and MR ratio of the TMR element depend greatly on the voltage applied to the TMR element. The MR ratio decreases with increasing voltage. Thus, a variation in the magnitude of the voltage applied to each of the TMR elements forming the bridge circuit causes the resistance characteristics of the TMR element to vary.
Hence, when the magnetization in the free layer rotates depending on the direction of the external magnetic field to cause the resistance value of each of the MR elements to vary, the magnitude of the voltage applied to the MR element varies. The resistance value of the TMR element further varies depending on the variation in the magnitude of the voltage.
Thus, the resistance value varies not only due to rotation of the magnetization in the free layer but also because of variation in the resistance value caused by the variation in the value of the voltage applied to the TMR element. As a result, the output waveform from the magnetic sensor deviates from a cosine or sine wave.
As described above, by using the magnetic sensor, the direction of the external magnetic field is calculated from the output value from the magnetic sensor based on the assumption that the output waveform will be shaped like a cosine or sine wave. Thus, the deviation of the output waveform from a cosine or sine wave may reduce the accuracy of the magnetic sensor.
As described above, the present inventors have found that the voltage dependence of the MR ratio of the TMR element may disadvantageously disturb the output waveform from the magnetic sensor, thus reducing the accuracy of the magnetic sensor.